The present invention relates to a charging generator controlling device which includes a voltage regulator for maintaining an output voltage of a conventional charging generator (alternator) at a predetermined value, and a current path switching circuit for switching current paths to a display lamp and a load in response to the output voltage of the charging generator, and including especially a protective circuit to protect the device from an excess current flow due to a short circuit.
First, a charging generator of the prior art will be described with reference to FIG. 1. In FIG. 1, reference numeral 1 designates a three-phase AC generator, which may be mounted on a vehicle (not shown) and which is driven by an engine (not shown). The AC generator includes three-phase star-connected armature coils 101 and a field coil 102. In FIG. 1, reference numeral 2 designates a full-wave bridge rectifier for full-wave rectifying the AC output of the generator 1, reference numerals 201 and 202 designate an output terminal of the rectifier and a ground terminal, respectively, and 3, a voltage regulator which controls the field current of the field coil 102 to maintain the output voltage of the generator 1 at a first predetermined value.
The voltage regulator 3 includes, as shown in FIG. 1, a surge absorbing diode 301 connected between the terminals of the field coil 102, Darlington-connected output transistors 302 and 303 for selectively interrupting the current in the field coil 102, a resistor 304 supplying base current for the transistors 302 and 303, a control transistor 305 for turning on and off the transistors 302 and 303, a Zener diode 306 used to detect an output voltage at the rectifier output terminal 201 of the generator 1 and which is rendered conductive when the output voltage reaches the first predetermined value, and resistors 307 and 308 connected in series to form a voltage divider circuit which divides the voltage at the rectifier output terminal 201. Further in FIG. 1, reference numeral 4 designates a battery, 5 a keyswitch, 6 a charge display lamp, 7 a load, and 8 a current path switching circuit.
The current path switching circuit 8 includes a first pair of switching transistors 801 and 802 connected in Darlington fashion for energizing the charge display lamp 6, a resistor 803 supplying base current for the transistors 801 and 802, a second pair of switching transistors 804 and 805 which are rendered conductive when the transistors 801 and 802 are nonconductive to thus energize the load 7, a resistor 806 which determines the base current of the transistor 805, a switching transistor 807 which is rendered conductive when the voltage at the rectifier output terminal 201 is lower than the first predetermined value but higher than a second predetermined value defining a power-generating state of the charging generator controlling device, a diode 808 which blocks current flow through the resistor 806 when the switching transistor 807 is nonconductive, a Zener diode 809 which is connected to the base of the switching transistor 807 and which is rendered conductive when the voltage at the rectifier output terminal 201 is higher than the second predetermined value, and a resistor 810 through which current flows to the Zener diode 809 when it is rendered conductive.
The operation of the above-described prior art device will now be described.
When the keyswitch 5 is closed to start the engine, a base current is supplied from the battery 4 through the keyswitch 5 and the resistor 304 to the transistors 302 and 303 to thus render the transistors 302 and 303 conductive. When the transistors 302 and 303 are rendered conductive, a field current is supplied from the battery 4 through the keyswitch 5, the field coil 102 and the transistors 302 and 303 so that a field magnetomotive force is generated.
Then, when the engine is started and the generator 1 is driven, AC outputs are induced in the armature coils 101 in a magnitude determined by the speed of rotation of the generator 1. The AC outputs thus induced are full-wave rectified by the full-wave bridge rectifier 2. If the output of the rectifier 2 is lower than the first predetermined value, the potential at the voltage dividing point of the divider circuit composed of the resistors 307 and 308 will be low, and therefore the Zener diode 306 will be maintained nonconductive and the field current will remain supplied.
The output voltage of the generator 1 increases with the speed of rotation. When the speed of rotation of the generator 1 increases to the point where the output voltage becomes higher than the first predetermined value, the potential at the voltage dividing point of the divider circuit also increases until the Zener diode 306 is rendered conductive. Accordingly, base current is supplied through the Zener diode 306 to the transistor 305 to render the transistor 306 conductive. When the transistor 305 is conductive, the transistors 302 and 303 are rendered nonconductive. As a result, the current flowing to the field coil 102 is interrupted, whereupon the output voltage of the generator 1 decreases.
When the output voltage decreases to the first predetermined value, the Zener diode 306 and the transistor 305 are rendered nonconductive again. As a result, the field coil 102 is energized and the output voltage of the generator 1 again increases.
The above-described operation is repeatedly carried out to maintain the output voltage of the generator 1 at the first predetermined value. The output voltage thus controlled charges the battery 4.
The operation of the current path switching circuit 8 will now be described.
Just after the engine is started and the generator 1 is driven, the voltage at the rectifier output terminal 201 will be lower than the second predetermined value. Therefore, the Zener diode 809 and the switching transistor 807 will be rendered nonconductive. As a result, base currents of the second pair of switching transistors 804 and 805 are so small that the transistors 804 and 805 are rendered nonconductive. On the contrary, the base currents which are supplied to the first pair of switching transistors 801 and 802 are large enough to render the transistors 801 and 802 conductive.
When the transistors of the first pair of switching transistors 801 and 802 are conductive, the charge display lamp 6 is energized and turns on. On the contrary, when the output voltage of the generator 1 is controlled at the first predetermined value, which is higher than the second predetermined value, the Zener diode 809 and the switching transistor 807 are rendered conductive. Accordingly, the transistors of the first pair of switching transistors 801 and 802 are rendered nonconductive, and base current is drawn through the resistor 806 from the second pair of switching transistors 804 and 805 to thus render them conductive. As a result, the charge display lamp 6 is turned off and the load 7 energized instead through the second pair of switching transistors 804 and 805.
The above-described prior art device is accompanied by certain disadvantages. Particularly, if the connecting node A between the charge lamp 6 and the load 7 is grounded, under the condition that the output voltage of the generator 1 is controlled at the fist predetermined value so that the load 7 is energized through the second pair of switching transistors 804 and 805, an excess current will flow through the second pair of switching transistors 804 and 805 causing damage thereto.
Another prior art device is known which further includes a switching circuit connected between the voltage regulator 3 and the current path switching circuit 8 in FIG. 1. This switching circuit includes a switching transistor, main electrodes of which are connected to the line extending from the keyswitch 5 to the high voltage terminal of the field coil 102, and an oscillator for periodically driving the switching transistor. This switching circuit operates to control the initial current flowing through the field coil 102 when the keyswitch 5 is closed.
Such a prior art device is also accompanied by the same disadvantages as mentioned above. Moreover, such a prior art device is accompanied by another problem in that the switching transistor in line can be damaged by an excess current if the field coil 102 is shorted or the high voltage terminal thereof is grounded.
An object of the present invention is thus to provide a charging generator controlling device which includes a protective circuit which eliminates the above-described problems accompanying prior art devices.